The invention relates to a slotted clip and method of forming the same and, more particularly, to an assembly of attached clips having a resilient cord inserted in slots thereof.
In the mattress and bedding industry, it has long been known to use clips for attaching border wires to coil springs. These clips have a fairly standard generally U-shaped construction and are formed from a blank of metal material so that the clips have a crown portion and leg portions that depend from either end of the crown portion. One of the leg portions has a generally bifurcated construction to provide a pair of leg projections that are spaced from each other, and the other leg portion is formed into a single projection that is aligned with the space between the bifurcated leg projections so that the projections do not interfere with each other as they are clinched about adjacent wires to be attached.
For storage, transportation and application purposes, several different ways to collate the clips into an assembly of attached clips have been utilized. Generally, some sort of flexible connecting member is utilized so that the clips are identically oriented in end-to-end fashion in a row for being loaded in either a hand-held application tool or a vertical rotating clipping (VRC) machine which cut individual clips from the assembly and clinches them about wires to be held together.
Both the hand-held application tool and the VRC machine have a piston driven plunger which has a knife edge that cuts the particular connecting member utilized to hold the clips in assembled form. Various types of connectors have been employed for keeping the clips together in a flexible manner so that the clips can be compactly wound into a coil for storage and shipping purposes, and so that they can be unwound and fed into the application tool. Flexible wire connecting elements are commonly used to interconnect U-shaped clips. Parallel wires are attached to the clips by welding to the tops of the clip crowns and are severed by the aforementioned knife edge on the tool plunger. One shortcoming of the flexible wire connectors is that they cause a dulling of the application plunger tool knife edge which requires that the plunger blade be regularly sharpened or replaced so that the tool can consistently separate the clips from the collated assembly for clinching individual clips about the wires.
It is also known to hold clips together in assembly by plastic cords held in slots formed in the crown portion of the clips, see e.g., published application GB 2,023,216 A; U.S. Pat. No. 5,303,821; and U.S. Pat. No. 5,564,564. Plastic cords have the advantage over their metal wire counterparts of not having as much of a dulling effect on the knife edge of the application tool plunger, and not requiring that the blade edge be as sharp in order to sever the plastic cord. Because the plastic cords use slots formed in the clips as opposed to being welded directly to the clip material, the challenge with plastic cords is how best to form the slots and properly hold the cords therein. In this regard, manufacturing and material costs are important considerations, given the additional step required to form the necessary slots in the clips and the generally higher raw material costs for the plastic material versus metal wire.
While it is known to use a friction or press fit of plastic cords into the clip slots, this type of connection in and of itself normally is less than desirable for withstanding the various tensile forces and bending and twisting forces to which the attached cord will be subjected, such as occasioned by the coiling and uncoiling of the collating clip assembly, without separation of the clips from the cord. Also, because of cost considerations, it is desirable to use the smallest diameter cord possible and so that the cord can be properly and easily severed by the application tool, while still maintaining the tolerances required for a consistent friction fit in the slot from clip to clip in the collated assembly. Another concern where very small diameter cords are used to be friction fit in clip slots is that the slots have to be also of a very small size which can cause problems when they are being formed. To form these small slots in a stamping or punching process, a relatively small sized punch is needed. Small punches that are not very robust may reduce punch life to a level that is not acceptable for high volume production of collated clip assemblies. Accordingly, the size of the plastic cord and thus of the slot in which it is friction fit is a compromise between a cord diameter that meets the desired cost criteria and allows for proper severing in the application tool, and an acceptable punch life for high volume clip assembly.
As previously discussed, there have been a number of prior clips which utilize special slot constructions to hold the plastic cord therein. It would be desirable to form the clip slots such that they hold the cord in the slots without requiring significant modifications to the manufacturing process. In most prior clips, their manufacture requires that additional clip material be bent or deformed for holding the cords in the slots of the clips, which generally calls for an additional manufacturing step, undesirably raising production costs. Accordingly, there is a need for a better formed clip slot for securely receiving plastic cords therein.
The small diameter plastic cords also tend to present a problem in keeping the cord at a circular cross-sectional shape within desired tolerances for being properly press fit in slots of the clips. With many plastic materials that meet the necessary cost criteria, there is a trade-off between achieving the desired cross-sectional shape and having the right tolerances for press fitting the cord into the slot.
Another factor in the design of the cord is the melt characteristics of the plastic material that is utilized. This can be important where the assembled mattress springs having the clips clinched thereon are tempered in a heat treat oven. If the heating is sufficient to melt the plastic of the cords in the clip slots, a problem arises if the plastic material beads together so as to create a bump over the top of the clips. Any small bumps like these are to be avoided, as they could create problems when the mattress material is applied over the assembled springs.
In accordance with the present invention, a slotted clip and a collated assembly of such clips that are connected by at least one plastic cord are provided with the cord or cords being securely and captively held in aligned slots of the clips. The cords are press fit into the slots and are held therein without requiring a separate manufacturing step or deformation of clip material to obtain a secure connection of the cord in the slots similar to many prior collated clip assemblies. More particularly, projections are provided in the slot so that when the cord is inserted and seated in the slots, the projections resist cord pullout therefrom. Further, in a preferred form, the slot having the projections is provided with burrs that are intentionally formed on the side walls of the slot for gripping the cord to resist shifting of the cord in the slots.
Thus, for example, when the blade edge of an application tool plunger becomes dull, operation thereof, rather than cleanly shearing through the cord, can tend to pull on the cord before it breaks off causing it to rise up in the slot. The projections in the slots of the present clips tend to keep the cord properly seated in the slot, and the burrs on the slot side walls resist the pulling action on the cord created by the dull application tool blade.
In a preferred form of the invention, a clip is provided having a body formed of metallic material and having an arcuate crown portion and depending leg portions with the clip having a width across the crown and a length transverse to the width. At least one slot is provided that extends lengthwise in the clip body and through the crown portion thereof to define a slot opening for receiving a resilient cord therein. Side surfaces of the slot extend vertically through the clip body and have a predetermined spacing in the widthwise direction therebetween. The predetermined spacing is selected to create an interference friction fit with a resilient cord that is press fit in the slot. At least one projection is provided at a predetermined position along the length of one of the slot side surfaces and which extends into the slot opening for maintaining a resilient cord press fit into the slot against shifting out from the slot.
In a preferred form, the slot side surfaces include burrs for gripping a resilient cord to resist shifting of a cord in the slot in the lengthwise direction.
A pair of identical parallel slots may be provided in the clip for correspondingly receiving a pair of resilient cords press fit therein.
Preferably, a pair of opposing projections are provided that extend into the slot opening from predetermined positions along both slot side surfaces toward each other. The slot includes opposite ends interconnecting the slot side surfaces, and the predetermined positions of the projections can generally be midway between the slot ends along the respective side surfaces.
To assist in severing of the cord by the application tool, the slot can be formed with either V-shaped ends or a thin edge provided thereat so that when the cord is seated at the ends of the slot, an area of weakness is created in the cord.
In another form of the invention, a collated assembly of attached clips each having a body of a metal material and including a crown and depending leg portions is provided. The clip assembly includes slots in the clip bodies having slot walls defining slot openings with adjacent clip bodies having slot openings that are arranged to be in alignment with one another in the clip assembly. A cord of resilient plastic material is inserted in the aligned slot openings of adjacent clips for holding the adjacent clip bodies together. Burrs are formed on the slot walls to grip the plastic cord to keep the cord substantially fixed in the slot openings of adjacent clip bodies in the clip assembly to hold the clips attached together in the collated assembly.
The slots can include substantially vertical side surfaces extending parallel to each other and spaced at a predetermined distance across the slot openings from each other. The plastic cord can have a generally circular cross-sectional configuration with a diameter sized to provide an interference friction fit between the parallel side walls.
The cord can be provided with areas of reduced diameter spaced therealong so that they are disposed between adjacent attached clips or, as discussed, these can be formed when the cord is inserted in the slots by the specially formed ends thereof for ease in severing of the cord to separate clips from the assembly.
The cord may be provided with an outer contoured surface that has a maximum diameter larger than the spacing between the slot side surfaces for providing a press fit despite minor variations in the diameter along the length of the cord. The contoured surface can be provided on one of a fluted cord, a splined cord, a twisted cord, and a helical cord. The helical cord can have one of a helical projection and a helical depression on its outer surface. In this manner, the cords of the invention do not require the tight tolerances of a cord having a circular cross-sectional shape for being press fit in the slots while still providing for proper severing characteristics.
Another alternative construction for the cord can be to provide it with a tubular configuration having an outer diameter sized to provide a press fit in the slot opening between the side surfaces thereof. The tubular cord reduces the amount of plastic material and thus the cost of the cord. In addition, the application tool blade does not have to go through as much plastic material, enhancing the severability thereof. Similar to the above-described cords having a contoured outer surface, the tubular cord can be provided with a larger outer diameter over cords with circular cross-sectional configurations without significantly hampering the cord insertion process.
Preferably, the cord plastic material is a polymer that is generally flowable at temperatures over approximately 450xc2x0 F. In this manner, when mattress springs having the clips applied thereon are tempered in a heat-treat oven, the severed cord section in the slot of an individual clip will melt and flow such as to the interfaces between the attached wires and coils rather than beading up. It has been found that one plastic material that meets these criteria is polybutylene terephthalate (PBT).
Another aspect of the invention is the provision of a die apparatus for forming slots in a blank strip of material. The die apparatus includes upper and lower portions that are moved toward and away from each other in a slot forming cycle. A punch is carried by the upper portion and includes a slot forming portion. A die opening is formed in the lower portion and has spaced sides sized to receive the slot forming portion of the punch therein for punching and separating portions of the material disposed over the die opening when the die portions are moved toward each other to form a slot in the strips substantially having the shape of the die opening. Projections extend into the die opening toward each other from the sides of the die opening to limit the pulling of punched and separated portions of the strip material back up with the punch as the die portions move away from each other. Accordingly, the projections of the die herein serve dual purposes of forming projections in the slots which assist in holding the attaching plastic cord therein as previously discussed, and enhance manufacture of the slots by limiting or preventing punched out slugs from being pulled back through the die opening with the punch as it is retracted therefrom.
The spaced sides of the die opening can extend substantially parallel to each other, and the projections can include a surface spaced from the respective die sides and extending substantially parallel thereto such that a first clearance is provided between the slot forming portion of the punch and the projection surfaces and a second larger clearance is provided between the slot forming portion of the punch and the sides of the die opening. Preferably, the first clearance between the punch slot forming portion and the projection surfaces is oversized for forming burrs on corresponding slot sides.
In another aspect of the invention, a method of forming a collated assembly of clips attached by a resilient cord is provided. The method includes feeding a section of a blank metal strip over a die opening in a slot forming die for punching slot openings in the strip, providing projections extending into the die opening from opposite sides thereof, driving a punch against the strip section and into the die opening to punch a slot opening in the strip section so that the slot has sides and projections substantially corresponding to the die sides and projections, retracting the punch from the die opening, restricting slugs of the strip punched therefrom from being pulled out from the die opening by the die projections as the punch is retracted, advancing the slotted strip section to a strip cutting and clip forming station with a succeeding section of the strip being brought into position over the die openings, cutting and forming a clip from the slotted strip section with the clip having crown and depending leg portions as the succeeding section of the strip has a slot opening punched therein, and inserting a resilient cord into slots of cut and formed slotted clips to attach the clips together in a collated assembly.